1. Field of the Invention
This invention relates to a method for effectively preventing (meth) acrylic acid from polymerizing during the course of distillation.
2. Description of the Related Art
It is generally known that acrylic acid has a nature to easily and spontaneously polymerize on exposure to light and heat. During the production of acrylic acid, therefore, various polymerization inhibitors such as hydroquinone, methoquinone, phenothiazine, copper dibutyl dithiocarbamate, p-phenylene diamines, and N-oxyl compounds are added either singly or in the form of a combination of two or more members to the reaction system of production with the object of preventing acrylic acid from polymerizing.
U.S. Pat. No. 4,021,310 discloses a method for preventing acrylic acid from polymerizing by using a polymerization inhibitor composed of at least one compound selected from the group consisting of hydroquinone, methoquinone, cresol, phenol, t-butyl catechol, diphenyl amine, phenothiazine, and methylene blue, at least one compound selected from the group consisting of copper dimethyl dithiocarbamate, copper diethyl dithiocarbamate, copper dibutyl dithiocarbamate, and copper salicylate, and molecular oxygen.
JP-A-51-98,211 discloses a method for preventing acrylic acid from polymerizing by using a polymerization inhibitor composed of a manganese salt such as manganese acetate, hydroquinone and/or methoquinone, and molecular oxygen.
GB 1127127 discloses a method for preventing acrylic acid from polymerizing by using a N-oxyl compound such as tertiary butyl nitroxide or 4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl.
Then, EP 695736 discloses a method for refining acrylic acid by distilling in an azeotropic dehydration distillation column an aqueous acrylic acid solution obtained by absorbing in water the reaction gas resulting from oxidizing propylene and/or acrolein with a molecular oxygen-containing gas or the liquid resulting from deriving the solution of aldehydes, which method effects the refinement by causing the bottoms of the distillation column to contain an azeotropic solvent at a prescribed concentration and consequently preventing the liquid in the distillation column from polymerizing.
A study made by the present inventors on the methods for preventing acrylic acid from polymerization by using polymerization inhibitors as described above has revealed that these methods fail to manifest a fully satisfactory effect in preventing polymerization under the conditions shown below.
In the production of acrylic acid by catalytic gas phase oxidation of propylene and/or acrolein with a molecular oxygen-containing gas, the acrylic acid-containing solution which occurs therein is distilled in a distillation column. Since this acrylic acid-containing solution is so composed as to contain impurities such as water, acetic acid, and acrolein, it is highly liable to induce polymerization of acrylic acid. When such a polymerization inhibitor as mentioned above is used at an ordinary application rate, it fails to produce a fully satisfactory effect in preventing polymerization. The solution during the course of distillation, therefore, is suffered to give rise to a polymer and the polymer renders protracted continuous operation of the distillation column difficult. In order for the continuous operation to last, it becomes necessary to use the polymerization inhibitor in a large amount. Thus, the polymerization inhibitor has not been fit for use in an actual apparatus.
The effect of preventing polymerization which is recited in EP 695736 is attained by causing bottoms in the azeotropic dehydration distillation column to allow presence of an azeotropic solvent at a prescribed concentration. Nowhere in this publication is found any mention purporting that the presence or absence of aldehydes in the aqueous acrylic acid solution supplied to the distillation column mentioned above affects the liability of this solution to polymerization during the course of distillation. In the examples cited therein, the distillation was performed only on aqueous acrylic acid solutions containing 0.3 wt. % of formaldehyde. The liability to polymerization of an aqueous acrylic acid solution containing a specific aldehyde at a specific concentration is mentioned nowhere.
The same problems are also encountered in the refinement of methacrylic acid.
The aforementioned prior patent publications disclosing methods for preventing acrylic acid from polymerization have absolutely no mention of the relation between the concentration of a specific aldehyde or ketone in the acrylic acid-containing solution fed into the distillation column and the effect of a polymerization inhibitor.
The present inventors have ascertained by their own study that the expected effect of preventing polymerization is not attained fully satisfactorily, depending on the concentration of a specific aldehyde or ketone in the acrylic acid-containing solution fed into the distillation column.
Thus, it is an object of this invention to provide a method for effectively preventing (meth) acrylic acid in a (meth) acrylic acid-containing solution from polymerizing during the distillation of the solution by the use of a polymerization inhibitor.
The present inventors have pursued studies on the prevention of acrylic acid in an acrylic acid-containing solution obtained by catalytic gas phase oxidation of propylene and/or acrolein from polymerizing during the distillation of the solution and, consequently, have found that the prevention of acrylic acid from polymerization is effectively attained by maintaining the total concentration of aldehydes of 2 to 4 carbon atoms and acetone in the acrylic acid-containing solution fed into a distillation column below a prescribed level. This invention has been achieved based on this knowledge. They have further found that this knowledge is applicable to the refinement of methacrylic acid.
To be specific, according to this invention it can provide a method for refining (meth) acrylic acid by a procedure comprising the steps of feeding a mixed gas obtained by catalytic gas phase oxidation of propylene and/or acrolein with a molecular oxygen-containing gas or a mixed gas obtained by catalytic gas phase oxidation of at least one compound selected from the group consisting of isobutylene, t-butyl alcohol, and methacrolein with a molecular oxygen-containing gas to a (meth)acrylic acid collection column, collecting a (meth)acrylic acid-containing solution from the mixed gas, feeding the (meth)acrylic acid-containing solution to a distillation column, and separating and recovering (meth) acrylic acid from the solution, which method is characterized by feeding the (meth)acrylic acid-containing solution to the distillation column while maintaining the total concentration of aldehydes of 2-4 carbon atoms and acetone in the solution at a level of not more than 2000 ppm based on the amount of (meth)acrylic acid.
The above and other objects, features, and advantages of the present invention will become clear from the following description of the preferred embodiment.